X-ray fluorescence analysis is a well-known method for investigating the material composition of a sample. X-rays from an X-ray source are allowed to irradiate the sample, which causes atoms of the sample constituents emit fluorescent radiation at characteristic energies. A detector receives the fluorescent radiation. By following the output of the detector it is possible to derive the spectral intensity of the fluorescent radiation. The relative numbers of received fluorescent photons at different energies reveal the proportions of different elements present in the sample.
In some X-ray fluorescence measurements it is important to know the absolute intensity of the incident X-rays that irradiate the sample. This is especially true when measuring soil or polymer samples, or other kinds of samples characterised by relatively low concentrations of heavy elements that are of interest, embedded in a matrix that consists of light elements. Since most practical X-ray fluorescence analysers use an X-ray tube as the source of incident radiation, the most straightforward way of finding out the absolute intensity is to make calibration measurements with known samples and to ensure that the X-ray tube will always be used under the same operating conditions (especially input voltage and current) as in calibration.
The generation of X-rays in an X-ray tube is fundamentally a stochastic process that involves a certain degree of statistic fluctuations. Additionally the electric circuit elements, however meticulously designed and controlled, that feed the input voltage and current to the X-ray tube, are inherently imperfect, which adds an unknown factor to the intensity fluctuations of the X-ray tube. It is also typical that after an X-ray tube has been switched on, it will take some time before the radiation intensity stabilizes to the value that has been observed in calibration measurements. This is a disadvantage concerning such XRF measurements that otherwise could be performed in a shorter time.
FIG. 1 illustrates schematically a known solution that enables monitoring dynamically the intensity of the incident radiation. An XRF analyzer comprises an X-ray tube 101, a detector 102 and a front plate 103. The device must have a solid outer cover that encloses all radiating parts, but here only the front plate is shown to better illustrate the concept. The front plate 103 defines a sample window 104, against which a sample 105 is to be placed. Radiation from the X-ray tube 101 irradiates the sample 105 through the sample window 104, and fluorescent radiation is measured with the detector 102. There is a thin wire 106 drawn across the empty space between the X-ray tube 101, the detector 102 and the sample window 104. The wire 106 is made of a material the X-ray fluorescence characteristics of which are well known. Fluorescent peaks of the wire material do not overlap with those of typical target materials. Some of the incident X-rays will hit the wire 106 and cause fluorescence in the atoms of the wire material. While measuring the fluorescence spectrum of the sample, the detector 102 also monitors the amount of fluorescent radiation coming from the wire 106. Since all other factors related to the fluorescence measurement of the wire 106 are constant, changes in the fluorescent radiation coming from the wire can only be caused by fluctuations in the intensity of incident radiation.
FIG. 2 illustrates schematically an alternative prior art solution. Here the front part of an X-ray analyzer device is seen from inside. The X-ray tube 101, the detector 102, the front plate 103 and the sample window 104 are similar to those in FIG. 1. Detectors 201 are located at the edges of the sample window 104. The intensity of the incident radiation is monitored directly by measuring that part of it that hits the detectors 201.
The solutions of FIGS. 1 and 2 share the inherent disadvantage that although they partly help to combat the overall intensity fluctuations over time, they do not take into account any spatial fluctuations in the X-ray beam. If a detector screen would be placed transversely across the X-rays emitted by the X-ray tube towards the sample window, and the two-dimensional intensity pattern detected by such a detector screen would be monitored, it would not be constant but would exhibit significant variation over time. As a specific example, we can imagine that at some times there may be a clear intensity maximum at the very center of the beam, whereas at some other time the intensity may be more evenly distributed or even have arbitrary peaks at the fringe areas. How accurately the solutions of FIGS. 1 and 2 will respond to such spatial fluctuations will depend heavily on how the spatial elements involved (the wire in FIG. 1, or the detectors in FIG. 2) will coincide with the arbitrarily located intensity maxima and minima across the beam.
An objective of the present invention is to present a method and an arrangement for dynamically compensating for variations in the intensity of incident radiation in an X-ray fluorescence analyzer, avoiding the drawbacks of prior art solutions.
The objectives of the invention are achieved by using material essentially transparent to X-rays to support a very small amount of marker material distributed across an area through which the incident radiation passes.
An analyzer device according to the invention includes a carrier that is essentially transparent to X-rays and disposed to spatially coincide with an essential part of the two-dimensional area of the sample window, and marker material responsive to X-rays by emitting fluorescent radiation, wherein said marker material is mechanically supported by said carrier and essentially evenly distributed across at least that part of the carrier that spatially coincides with the two-dimensional area of the sample window.
An analyzer arrangement according to the invention comprises a carrier, at least a part of which is essentially transparent to X-rays, and marker material responsive to X-rays by emitting fluorescent radiation. The carrier is movable to a first location in which at least a portion of said part of said carrier spatially coincides with an essential part of the two-dimensional area of the sample window. The marker material is mechanically supported by said carrier and essentially evenly distributed across at least that portion of the part of the carrier that in said first location spatially coincides with said two-dimensional area of the sample window.
The invention is also directed to a measurement method, which comprises:                placing marker material, which is responsive to X-rays by emitting fluorescent radiation, so that the marker material is between an X-ray source and a sample and also between the sample and a detector and distributed essentially evenly across a substantial part of the two-dimensional area of a sample window that separates the sample from said X-ray source and said detector;        using said X-ray source to irradiate the sample with incident X-rays through the sample window;        receiving fluorescent radiation from the irradiated sample and from the marker material with said detector and measuring the intensity spectrum of the received fluorescent radiation;        
using the measured intensity spectrum to determine a correction factor, which comprises at least one of:                an indicator of the intensity of fluorescent radiation received from the marker material, and        an indicator of the location of a fluorescent radiation peak from the marker material on an energy channel; and        using said correction factor to process the measured intensity spectrum of fluorescent radiation received from the sample, thus producing a corrected intensity spectrum.        
The idea of placing some marker material to the beam of incident radiation, like the wire used in some prior art solutions, is good as such. However, one should note that all such analyzer capacity, which is used to detect fluorescent radiation coming from the marker material, chips away at the useful capacity that is available for detecting actual fluorescence from the irradiated sample. Thus it would not be viable to add more wires to such a known solution to increase spatial coverage, because each wire must have a certain finite thickness in order to avoid breaking, and the overall amount of marker material in the beam would quickly become excessive.
However, the amount of marker material does not need to be very large if the marker material does not need to mechanically support itself. If some other material that is essentially transparent to X-rays is used as the mechanical support, it suffices to coat and/or dope the carrier material with such a small amount of marker material that even an essentially even distribution of marker material can by achieved across the whole area through which incident radiation passes to the sample.
The mechanical support for the marker material is most advantageously a window foil in the sample window, but also other possibilities exist.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.